1. Field of the Invention
The present invention relates to a method of elongating a glass preform so that an optical fiber having air holes extending in the axial direction of the fiber can be produced by further drawing.
2. Description of the Related Art
Examples of an optical fiber having air holes extending in the axial direction (longitudinal direction) of the fiber are so-called holey fibers and photonic crystal fibers. Such an optical fiber having air holes extending in the axial direction of the fiber is hereinafter referred to as a “microstructured optical fiber”. It is possible to make the characteristics of the microstructured optical fiber superior to those of an optical fiber having no air hole, since a difference between the mean refractive index of a core region and that of a cladding region can be controlled by controlling the size and arrangement of the air holes in a cross-section perpendicular to the fiber axis. For example, the microstructured optical fiber can exhibit wavelength dispersion of a larger absolute value than that of an optical fiber having no air hole, and such a microstructured optical fiber is expected to be applied to dispersion compensation.
A known apparatus for elongating the glass preform has a function to control the quantity of heat applied to a heating means so as to attain a uniform outer diameter of an elongated glass preform (for example, refer to Japanese Examined Patent Application Publication No. 6-24996 and Japanese Unexamined Patent Application Publication No. 5-97459). The use of such an elongation apparatus causes in some cases excess shrinkage of the air holes in the glass preform from which a microstructured optical fiber is to be produced. The excess shrinkage of the air holes makes it impossible to realize a characteristic to be achieved by the microstructured optical fiber, for example, wavelength dispersion of a large absolute value.